This invention relates to a solvent-soluble polyimide and a process for producing the same.
It is known that aromatic polyimides excellent in heat resistance can be obtained by polycondensation of aromatic tetracarboxylic acid anhydrides or derivatives thereof and aromatic diamines. More in detail, a polyamide-acid is first synthesized, followed by curing at high temperatures for imidization to yield a polyimide. As a typical example, an aromatic polyamide-acid is synthesized from pyromellitic anhydride and diaminodiphenyl ether. But such an aromatic polyamide-acid should be cured at such a high temperature as 300.degree. C. or higher for imidization. The thus obtained aromatic polyimide is generally insoluble in a solvent. It is also known that a polyimide can be produced from an alicyclic dibasic acid and a diamine via polyamide-acid (Japanese Patent Unexamined Publication No. 60-135430). But this reference is quite silent on the production of a solvent-soluble polyimide.
On the other hand, it is proposed in Japanese Patent Unexamined Publication No. 59-179623 to produce a solvent-soluble polyimide by properly selecting a special combination of monomers. But according to this method, the resulting aromatic polyimide film is colored in a yellowish brown color and poor in light transmittance. Thus, a polyimide solution having good light transmittance has been desired in fields wherein light curing or light transmittance has an important meaning.